A typical vacuum type circuit interrupter comprises a pair of relatively movable contacts which can be separated, or opened, to establish a gap therebetween across which an arc is formed. This arc vaporizes some of the electrode material to create a local atmosphere through which current flows until about the time a natural current zero is reached. When this current zero is reached, the arc vanishes, and a recovery voltage transient builds up across the arcing gap. If the gap is able to withstand this recovery voltage transient the arc is prevented from reigniting and interruption is completed.
The ability of the gap to withstand the recovery voltage transient is largely dependent upon the extent to which the gap is free of ionized arcing products when the recovery voltage transient is applied. If, for example, the gap were entirely freed of arcing products, the vacuum in the interrupter would make available a very high dielectric strength to withstand the recovery voltage transient. The extent to which the gap is free of ionized arcing products when the recovery voltage transient is applied depends to a large degree on the ability of the interrupter to condense these hot arcing products prior to the application of the recovery voltage transient. The more completely the interrupter condenses the arcing products, the more likely it is that the gap will succeed in withstanding the recovery voltage transient.
During low current interruptions, the interrupter ordinarily has no difficulty in condensing the arcing products with sufficient rapidity and completeness to withstand the recovery voltage transient. However, the higher the current being interrupted, the greater the volume of arcing products which is generated, and the more difficult it becomes for the interrupter to condense these products in a time which permits interruption of the recovery voltage transient. Generally, it has been determined that when the interrupted current enters or exceeds the range of 10,000-20,000 amperes, large quantities of arcing products are generated and the ability to withstand the recovery voltage transient is diminished.
It is known in the art that by establishing an axial magnetic field in the gap of the interrupter during current interruption, the arc is diffused so as to minimize generated arcing products. U.S. Pat. No. 3,321,599 to T. H. Lee, assigned to the assignee of the present invention, is representative of art which uses a coil disposed external to the envelope of the interrupter to generate such a field. In pertinent part, Lee shows a coil disposed circumferentially about the vacuum envelope of an interrupter and extending substantially the length of the envelope. The coil is connected with a first of the interrupter contacts so as to be in series with the interrupter and hence the gap established therein when current is interrupted. Upon separation of the contacts to interrupt a current, the coil establishes an axial magnetic field in the gap. This axial magnetic field diffuses the arc so as to reduce arcing products and to permit the interruption of a substantially higher current than is possible in prior art interrupters.
Typical vacuum interrupter envelopes, such as the envelope shown in Lee, include a pair of electrically conductive end plates spaced by a cylindrical, electrically insulating wall. Each of these end plates is at the electrical potential of a respective one of the interrupter contacts, and the possibility of a potential breakdown or "strikeover" exists between the end plates through the relatively low-dielectric atmosphere outside the envelope. A coil of the type shown in Lee, being connected to a first of the contacts, is thus at the common potential of that contact and the end plate associated with that contact. The present inventors have discovered that when a coil is disposed outside of the envelope and for substantially the length of the envelope as in Lee, an undesirable possibility exists for a potential breakdown between that coil and the second end plate. Because the coil is at the potential of the first end plate and disposed in close proximity with the second end plate, the strikeover distance between the end plates is decreased to the distance between the closest point between the coil and the second end plate. The coil must thus be heavily insulated to prevent this electrical breakdown. This insulation increases both the complexity and the cost of the interrupter.
It would thus be desirable to provide a vacuum interrupter including a coil disposed so as to develop an axial magnetic field for diffusing an arc, but which does not substantially decrease the strikeover distance between the end plates. Such a vacuum interrupter would be capable of interrupting a high peak current without the additional complexity and cost of a highly insulated, lengthy coil. It would be further desirable if a method could be provided for modifying existing vacuum interrupters to provide the advantages of the present invention.